The only estimate that has been made was an illustrative sensitivity test included in Air Traffic Forecasts for the United Kingdom produced by DfT in 2000. This considered the impact of fares remaining constant in real terms throughout the 1998–2020 period in place of the central assumption of a 1 per cent. annual decline. This sensitivity test showed that passenger traffic at UK airports in 2020 would fall from the central forecast of 401 million to 301 million. However, this was based on the very simple assumption that a 10 per cent. rise in fares would lead to a 10 per cent. fall in demand. In practice, rising real incomes at home and overseas are likely to have a substantial positive impact on the demand for air travel.